The Phase Rule and Different Components

Mais Mazin Assistant Lecturer in Pharmaceutics

• Mixtures are a physical combination of two or more substances or elements. These substances that are combined together have no chemical interactions with each other and hence retain their inherent chemical properties. However, parameters like the melting point or boiling point are affected by the formation of a mixture.

Characteristics of a Mixture • As none of the components of a mixture are chemically

interacting with each other, separating components from a mixture is easy.

• The chemical properties of components are not altered in a mixture.

• A mixture can be formed by combining substances in any state of matter. Example: A combination of salt in water is a mixture.

• In a mixture, the components need not be present in definite concentrations.

• The formation of a mixture is not associated with any change in energy or enthalpy, unlike the formation of a new compound

Different Mixtures: • Based on the states of matter of individual

components (dispersion phase and dispersion medium) of a mixture. The mixtures can be:

• Solid in Solid • Solid in Liquid • Solid in Gas • Liquid in Liquid • Liquid in Gas • Liquid in Solid • Gas in Gas • Gas in Liquid • Gas in Solid

Types of Mixtures:

• There are two broad classifications of a mixture: • 1. Homogeneous Mixture: • Homo: the meaning of the term is ‘same or uniform’. • A homogeneous mixture is a type of mixture in which the

components are uniformly distributed. • None of the components involved in the homogeneous

mixture can be detected or observed separately. • The homogeneous mixture is always confused with a

substance, but the main difference is proportion. In contrast, homogeneous mixtures have no fixed composition and contain indefinite amounts of any component.

• A solution is a great example of a homogeneous mixture. When a solute completely dissolves in a solvent, the mixture formed is a solution. Here the solute is the dispersed phase, and the solvent is the dispersion media. Example of solution: salt+ water

• 2. Heterogeneous Mixture

• Hetero: This term stands for different.

• heterogeneous mixture can be defined as a mixture with no uniformity.

Individual components of a heterogeneous mixture can be discreetly observed. As the components are visible, it is easier to separate the individual components of a heterogeneous mixture than the homogeneous mixture.

• Colloids and suspensions are examples of heterogeneous mixtures.

Summary: In order to summarize the two types of mixtures: homogeneous and heterogeneous,

let us understand their differences.

Phase Rule : Gibbs phase rule may be stated as follows : "In a heterogeneous system in equilibrium, the

number of degrees of freedom plus the number of phases is equal to the number of components plus two".

Mathematically, F + P = C + 2 or F = C – P + 2 where F = number of degrees of freedom C = number of components P = number of phases 2 = additional variables of temperature and

pressure besides the concentration variables

phase :-is homogeneous physically distinct portion of the system which is separated from other parts of the system by bounding surfaces.

Number of component : is the smallest number of constituents by which the phase of equilibrium system can be expressed as a chemical formula or equation.

Degree of freedom: is an independent physical parameter in the formal description of the state of a physical system

one component system

• Water system consists of equilibrium

It is a one component system because the composition of the three phases can be expressed as water.

. Phase Diagram of Water

Some important features of Water System:

• (i) Possible phases : Ice (s), Water(l), Vapour (g)

• (ii) Curves: three stable curves (a) OA ( Vapour pressure

• curve, Water Vapour)

• (b) OB ( Sublimation Curve, Ice Vapour)

• (c) OC ( Melting point curve, Ice Water )

• iii) One metastable curve OA’ (Vapour pressure curve of

super cooled water)

• (iv) Areas : Three areas representing ice, water and vapour.

Triple point (O): Where all the three phases are in equilibrium.

• Application: – A liquid such as water in equilibrium with its vapor ( we have 2

phase system) – F=1-2+2=1. – By stating temperature, the system is completely defined because

the pressure under which liquid and vapor can coexist is also fixed. – If we decide to work under a particular pressure, then the

temperature of the system is automatically defined: – The system is described as univariant.

• Application: – When we have a liquid water, vapor and ice – Phase rule states that the degrees of freedom = 1-3+2=0 – There are no degrees of freedom, if we attempt to vary the

conditions of temperature or pressure necessary to maintain the system, we will lose a phase.

– The combination is fixed and unique. – The system is invariant.

Examples (i)Sulphur system (a)monoclinic sulphur, (b)rhombic sulphur (c)liquid sulphur (d) sulphur vapour. (C = 1; P=4)

(ii) Water system solid,liquid and vapour (C=1 ; P = 3) . (iii) Salt + water system Certain salts are capable of existing as hydrates with different number of water molecules of crystallization. The system is a two component.(C=2 , P = 1)

Two component systems containing liquid phases

• Miscible systems such as water and ethanol.

• Immiscible systems such as water and mercury.

• Partial miscibility (or immiscibility) such as phenol and water.

Phenol and water. – These liquids are partially miscible in each other.

– At certain ratios the liquids are completely miscible and at

others they are immiscible. – The 2 degrees of freedom of this mixture are temperature and

concentration (condensed system). – The maximum temperature that these two liquids can exist in

a 2 phase system is 66.8°C. – This is called the critical solution temperature or the upper

consolute temperature. – In this system above 66.8°C all combinations of phenol and

water will be completely miscible and will be one phase.

Figure – Temperature-composition diagram for the system consisting of water and phenol

• The curve gbhci shows the limits of temperature and concentration within which two liquid phases exist in equilibrium.

• The region outside this curve contain systems with only one liquid phase.

• Starting at the point a, a system with 100% water and adding known increments of phenol to a fixed weight of water while maintaining the system at 50oC will result in the formation of a single liquid phase (water with dissolved phenol).

• This continues until point b.

• At point b, a minute amount of a second phase appears.

• The concentration of phenol and water at which this occurs in 11% by weight of phenol in water.

• Analysis of the second phase, which separates out in the bottom, shows it to contain 63% by weight of phenol in water.

• This composition corresponds to point c in the diagram.

• As we prepare mixtures with increasing quantities of phenol (we proceed from point b to point c), the amount of the phenol-rich phase (B) increases and the amount of the water-rich phase (A) decreases.

• Once the total concentration of phenol exceeds 63% at 50oC, a single phenol-rich liquid phase is formed

• The line bc drawn across the region containing two phases is termed the tie line.

• All systems prepared on a tie line, at equilibrium will separate into phases of constant composition, these phases are termed conjugate phases.

• Any system represented by a point on the line bc at 50oC separate to give a pair of conjugate phases whose compositions are b and c.

• The relative amounts of the two layers or phases vary.

• If we prepared a system containing 24% by weight of phenol and 76% by weight of water (point d), at equilibrium we will have two liquid phases.

• The upper one (A) has a composition of 11% phenol in water and the lower one (B) has a composition of 63% phenol in water.

• Phase B will lie below phase A because it is rich in phenol which has a higher density than water.

• Upper consolute temperature: All combinations above this temperature are completely miscible (one phase).

• Lower consolute temperature: below which the components are miscible in all proportions.

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